Investment composition



PERCENT THERMAL EXPANSION July 1,1941. R NElMAN 2,247,585

INVESTMENT COMEOSITION Fil d Jan. 16, 1939 0 200 400 600 800 I000 |200 I400'F TEMPERATURE lNVE NTOR' ffoberf Ne/mcm.

' and cooling to the solid state.

Patented July 1, 1941 UNITED STATES PATENT OFFICE v INVESTMENT COMPOSITION Robert Neiman, Louisville, Ky., assignor to The Columbus Dental Manufacturing Company, Columbus, Ohio, a corporation of Ohio Application January 16, 1939, Serial No. 251,275 1 Claim. (01. 22-188) curately to the form and dimension of the cavity to be filled.

In the reconstruction of a tooth or a group of teeth, the dentist prepares the cavity in a skillful manner, fills this cavity with warm wax, carves wax down to desired shape, removes same, and attaches a sprue. wax is then i completely encased in a cementitious mass of a mixture of investment with water. Upon setting to a hard mass, the practitioner proceeds to remove the sprue and places the mold in an oven or furnace. As this mold is heated the wax melts and then volatilizes, and upon continued heating the wax residues-of carbon are gradually decomposed and volatilized, the latter process requiring a temperature in the neighborhood of 1100 F. Between this temperature and 1300" F. is the generally desired mold temperature for good casting. Upon reaching this temperature the mold is removed from the furnace and the alloy is cast into the cavity left by the disappearing wax pattern.

" In this casting process the exact size of the desired inlay or the like is not produced, due to the casting shrinkage of the alloy in solidifying This shrinkage is approximately 1.25% (linear) and obviously must be'counteracted by a similar degreeof expansion of the mold, on heating, in order to reproduce a casting of a predetermined dimension. Due to various conditions and modes of investment. An investment meeting these desirable and necessary properties will give the skilled practitioners. simple, and sure method of making artistic and very accurate castings.

The invention should:

First-Have a thermal expansion (room temperature to approximately 1300 F.) in the neighborhood of 1.25% for a mix of painting or pouring consistency.

Second.---Attain nearly all of this expansion at 1150 F. with very little additional expansion to 1300 F.

Third-Have practically no variation in thermal expansion when mixed with an amount of water from 7% above to 7% below, the amount prescribed by the manufacturer, a leeway of 14% approximately.

Fourth.-Expand at a fairly even rate throughout the entire heating range, the expansion preparing the wax pattern the exact degree of the profession-has long desired in a commerciali heri ng film on the casting.

curve never rising very much above the practically straight line curve ofthe usual metallic containereor ring of equal expansion. I

Fifth-Incorporate readily with: water to a smooth creamy consistency which can be easily and readily applied to every nook and crevice of the wax pattern and will not fall away from the wax due to its slippery nature.

Sixth-Possess enough setting expansion to make an accurate reproduction (about .1% and then an additional amount of expansion to aid in compensating for shrinkages of wax pattern, when using the direct method, of .1 to 25%.

Seventh-Set rapidly to a hard cementitious mass which will withstand high temperatures .and rapid heating without cracking or distortion.

Eighth-Retain this hardness .for some time so that it may be used and handled, even the next day, if necessary, without undue danger of easy fracture of delicate parts.

Ninth-Possess" adequate strength when, hot to safely resist the sudden inrush of molten jalloy when cast under usual pressure; and preferably to possess enough strength after quenching in water, to permit easy removal in toto rather than to disintegrate and leave a tenaciously ad- Tenth.-Provide a mold physically very smooth, and of such chemical makeup as to prevent undue oxidation and sulfurization of the embedded alloy, and thus produce a smooth,

dense, casting without discoloration.

Eleventh-Not change materially on standing or shipping due to the settling out of unduly'heavy constituents such as powdered heavy 2 metals,fnecessitating packing under pressure. Also possessing constituents of a similar chemical and physical nature to give a homogeneous mass when mixed with water and also to withstand storage under usual conditions without undue disintegration and separation of ingredients. The principal ingredients of, what may called simple investment compositions, as heretofore used, has been a mixture of plaster, knownotherv of the forms of calcium sulfate, hydrated 4 or dehydrated, which set with water 'or'watery solutions to a hard mass, generally acts as they binder. The filler or aggregate consists of any refractory material which aids the mold in withstanding the high temperatures. It is desirable to use a filler which will also aid in giving a high thermal expansion. The most desirable filler in silica in its three principal modifications: quartz, tridyrnite, and cristobalite. Quartz is the cheapest and most universally used. Obviously any other filler'may be used that will contribute the 'desired properties, such as aluminum oxide, chromite, magnesium oxide, and other oxides as well as silicates, clays, and the like. To give the desired setting and hardening properties accelerators and retarders of a wellknown and commercial nature are added. Small amounts of graphite, and sometimes a little (fraction of a percent) clay, or the like are added to aid in producing a smoother mold and thereby smoother castings.

It is well known that mixtures of plaster and silica will not give the desired thermal expansion nor satisfy many of the other specified requirements. In the endeavor to meet the desired specifications various ingredients have been added to the plaster and silica mixtures with varying degrees of success. Cristobalite, for example, when added in large amounts (50 to 10%) gives a material of high thermal expansion and meets many of the above specifications. This composition does not, however, meet, or meets but partially, the third, fourth, fifth, seventh, ninth, and tenth requirements. In a composition in which sodium (potassium or lithium) chloride is added the eighth, ninth, tenth and eleventh requirements are lacking. Metallic ingredients when added fail to meet the second, third, tenth and eleventh requirements. Boric acid has also been added to the basic composition but is required in good amounts such as 3 to 5% in order to give adequate expansion, but the composition is still lacking in'the third, fifth, and tenth requirements. I

It is, therefore, the object of this invention to "provide the profession, and anyone desiring an artistic and accurate casting made, principally,

by the disappearing wax pattern method, with investment compositions which will meet the above requirements to a greater degree than anything now known or available.

Other objects and advantages of this invention should be readily apparent by reference to the following specification considered in conjunction with the accompanying drawing forming a part thereof, and it is understood that any modification may be made within the scope of the appended claims, without departing from or exceeding the spirit of the invention.

vestment of 1.1%.

This application is a continuation in part of my application Serial No. 62,801, filed February 7. 1936, for Improvement in investments.

The drawing depicts a pair of graphs showing the expansion of an investment compound when heated under identical conditions, mixed with sameproportion of water, and which compounds are respectively with and without one of the ingredients which may be added in the pursuance of my invention.

Simple noted, are well known and have been used for many years. The general composition of these materials is from 20 to 50% plaster and to 50% silica. The increase in silica content produces an increase in expansion but a decrease in strength. The proportions may be varied even beyond these limits depending upon the exact technic used in producing the castings.

In the drawing the reference numeral l0 indicates the thermal expansion (and contraction) of a mixture of 44% plaster with 56% quartz and indicates a thermal expansion of 54% between room temperature (70 F.) and 1300 F. This investment was made with a water to powder ratio (W/P) of .26, that'is 26 grams of water to grams of the powder mixture. I An increase of the W/P causes a material decrease in the thermal expansion of this type of mixture.

It has been found that by the addition to the above simple investment compositions .of small amounts of the chlorides of the alkaline earth metals, known in chemistry as the metals of the fourth group in' qualitative analysis (group IV) or the ammonium carbonate group, and including the chlorides of calcium, -barium, strontium, and magnesium, that the expansion quality of the investmentwill be materially increased. As shown by the second curve in the graph and indicated by the reference numeral II, the addition of 1.0% of any one of these chlorides for 1.0% of the quartz and maintaining the W/P ratio constant at .26, will cause an expansion of this in- Even small amounts of these chlorides will cause an appreciable increase in thermal expansion which continues to increase as the amount is increased. The upper limit of the amount of chlorides is perhaps 5% and even more depending upon the amount of plaster used and upon the desired expansion and strength. .I-Iow ever, in most investments that I have. tried, the

greatest increases in expansion occur up to 2% of the chlorides while the further increase resulting from the use of more than 2% is less proportionately. It is not desired to limit the composition to the inclusion of any certain specific amount as this depends upon the other ingredients, but a very good product can be made by the inclusion of 1% as shown by the graph. An increase in expansion will result if the silica content i increased and vice versa. Variation in the W/P ratio, however, no longer aifects the thermal expansion so markedly as heretofore and will enable the practitioner who does not take advantage of the modern measuring devices to investment compositions, as above other hand,- an investment material lar results 'are noted if these chlorides are dissolved in the water that is added to the usual simple investment compositions consisting of dry ingredients since the resulting mixture is the same.

Some investment materials are being marketed with cristobalite as the siliceous refractory material, with a plaster binder. Inasmuch as cristobalite in and of itself has an expansion of 2% when heated from room temperature to 1300 F., an investment material containing about 70% cristobalite and the balance a binder will have an expansion of about 1.25% from room temperature to 1300 F. I have found that by the use up to 5% of one or more of the above-mentioned chlorides in such a cristobalite investment, the expansion of the investment may be raised to as high as 2% from room temperature of 1300" F. Such an investment material is within the scope of my invention. Thus, it is within the scope of my invention to provide an investment composition containing from to 50% plaster and from 80 to 50% cristobalite.

While such an investment material may be desirable under certain conditions, it has some drawbacks, such as an unduly abrupt expansion upon subjection to the temperatures indicated and such as the necessity of using an unduly large amount of water for the mix with a consequent weakness of a mold made therefrom. Likewise, such an investment composition is relatively costly. However, I have found that by substituting certain amounts of quartz for the cristobalite, the abrupt expansion may be largely obviated, the required amount of water materially decreased and, at the same time, the cost of the investment composition may be materially reduced. Likewise, the resulting composition will give an increased mold strength.

It should be remembered that cristobalite has an inherent expansion quality of about 2%, quartz about 1.5% and trydimite about 1.3%, all from room temperature to about 1300 F. Therefore, other conditions being equal, the greater the percentage of cristobalite present, the greater will be the expansion due to the filler. Most any combination of these forms of silica may be used and it is not entirely necessary that these materials be used as distinct entities, since the grains of the siliceous material may contain a mixture of one or more of these forms of silica, dependin upon the degree of calcination. In these various forms of investment composition which contain cristobalite it is preferable that the total filler be present in an approximate range' of 50 to 80% of the total investment composition.

Equal or very nearly equal results, as to properties, may be produced in these cristobalite investment compositions by the use of any one or more of the chlorides of group III or group II of qualitative chemical analysis, .or by ammonium, rubidium or caesium. More specifically, it will be understood that by the use of any of the chlorides here described, the reduction inexpansion due to the substitution of an appreciable quantity of quartz for cristobalite may be overcome or substantially minimized. 'For example, an investment material containing 70% cristobalite and plaster will expand 1.25% from room temperature to 1300 F. (W/P=.38) while 69% eristobalite and 30% plaster plus 1% of one of the above .chlorides such as ammonium' chloride will expand; 1.45% from room temperature to 1300 F. (W/P.=.3 8). On the containing 30% cristobalite, 40% quartz and 30% plaster plus 1% of one of the above chlorides, such as ammonium chloride, will expand 1.25% from room temperature to 1300" F. (W/P=.325).

From this it will be seen that by using a small percentage suc has 1% of one of the above-mentioned chlorides it will be possible to substitute quartz for 40% of the cristobalite without loss in expansion. This will also ameliorate the sudden or abrupt expansion which commonly occurs when cristobalite is used in a higher percentage. Furthermore, it will reduce the W/P ratio which will increase the strength of molds made from the investment. In addition, the appearance of castings made by the use of a mixture containing cristobalite, quartz, plaster and one of the chlorides mentioned in the percentages indicated will materially lower the cost of the investment material and will result in a markedly improved appearance of the castings produced therefrom. In these compositions containing cristobalite, I

. may use any one or more of the chlorides-menbarium chloride as an example and state that by the use of this material an investment can be made which -will meet all of the necessary and desirable requirements set out above. Besides imparting marked increase in thermal expansion it forms a plastic mass with silica, plaster,-and water, that is very smooth and creamy in its form for application to the wax, thus producing smooth and very desirable reproductions. This composition hardens to a cementitious mass which retains its strength for days, withstands rapid heating, and high temperatures. It does not adhere to castings and produces smooth,

dense castings of such brightness that necessitate.

no pickling in acids, and subsequently very little buffing or grinding. producing beautiful castings it is ideal and the casting produced is often as clean and bright as, the original alloy, a property long desired in investment compositions. Since this barium chloride salt is by nature similar in density to the density of silica'and plaster it will not settle out on storage and will maintain a finemixture for a long time. i

It has also been found that a marked increase in expansion may be secured by the use of a comparable amount of the chlorides of groups III and II of qualitative analysis. The best results are obtained from the chlorides of group III metals in the following descending order: aluminum, chromium, nickel, iron, manganese, cobalt; and of the following descending order in group II: copper, tin, antimony, lead, bismuth, cadmium. It has also been found that the chlorides of ammonium. and the rare rubidium and caesium also increase the expansion markedly. While the chlorides of the alkaline earth metals produce the best results in meeting the desired requirements of an investment composition, it is obvious that should these not be available, or should the cost factor change materially due to economic and industrial changes, or should all In this last property of namely, calcium chloride.

.ting expansion, and accelerate the setting time.

From among the various chloridesit is therefore possible touse one or a combination of two or more as to give an investment possessing each of the above properties in the desired amount or degree. It may also be possible to secure the desired results by the use of other materials or minerals which will react to produce any of the desired chlorides. Thus hydrochloric acid has been found to increase the thermal expansion of silica-plaster investments nearly as much as the other chlorides, 'and this is no doubt due to the formation of one of the specified chlorides; Any other material containing chlorine which can be liberated to reactwith the plaster or other added ingredients may also produce the same beneficial results. The addition of the specified chlorides to investments containing other expanding ingredients will generally add its influence in aiding the increase in thermal expansion as well as adding alpha gypsum.

terms plaster, binder/1 filler and investment are to be interpreted as including:

Plaster.--Shall be construed to include any or the forms of calcium sulfate, such as the ex- ,mit the successful casting of metals and alloys especially those of the precious metals; The

usual investments contain-20 to 50 per cent of binder and 50 to so per cent of filler. The types and combinations of filler and binder shall further include only those that will have their'thermal" expansion increased by the addition of the chlorides herein set forth..

When definite examples of expansion values are given, the plaster used was in the form of Having thus described my invention, claim is:

An investment composition comprising silica, plaster of Paris and from 0.25% to 3% of ammonium chloride. r

ROBERT NEIMAN.

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